10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Sensitivity of Bare Black Carbon MAC and AAE to Morphological Parameters, Primary Particle Polydispersity, and Refractive Index in the Visible and Near-Infrared
FENGSHAN LIU, Jerome Yon, José Morán, Andrés Fuentes, Joel Corbin, Prem Lobo, Gregory Smallwood, National Research Council Canada
Abstract Number: 534 Working Group: Aerosol Physics
Abstract The mass absorption cross section (MAC) and Ångström absorption exponent (AAE) in the visible and near-infrared spectrum are important optical properties of freshly emitted black carbon (BC) (also called soot) particles. These optical properties are required in climate modeling and in the real-time BC mass concentration measurement through conversion of BC absorption coefficient measured by optically based instruments, such as the cavity-attenuated phase shift (CAPS) PMSSA monitor and the photoacoustic extinctiometer (PAX). The basis of conducting BC mass concentration measurements using CAPS and PAX and other similar instruments is the assumption that MAC and AAE are fairly robust. Although the effects of certain parameters, such as the fractal dimension Df, aggregate size Np, and refractive index m, on the radiative properties of bare BC aggregates have been investigated previously, there is a lack of systematic understanding on how morphological parameters, primary particle size distribution, and refractive index affect MAC and AAE of BC aggregates.
In this study the optical properties of BC aggregates, especially the absorption cross sections required to derive MAC and AAE, are computed for numerical fractal aggregates generated using the diffusion-limited cluster aggregation (DLCA) algorithm and tunable cluster-cluster aggregation (CCA) algorithms when the effect of a specific parameter is investigated. The morphological parameters (Df, kf, dp, and Np) considered in this study are relevant to combustion-generated BC particles and different refractive indices available in the literature in the visible and near infrared spectral region (400 to 1100 nm). In addition, the potential importance of primary particle polydispersity is also investigated by generating fractal aggregates formed by lognormally distributed primary particles. The optical properties of BC fractal aggregates are calculated using the generalized Mie-solution method (GMM) and the discrete dipole approximate (DDA). The objective of this study is to quantitatively understand how the morphological parameters, primary particle size distribution, and the refractive index affect the theoretical MAC and AAE.
Since the degree of internal multiple scattering within aggregate is dependent on all the parameters considered, i.e., the morphological parameters, primary particle size distribution, refractive index, and wavelength, it is anticipated that MAC and AAE will display different degrees of departure from the Rayleigh-Debye-Gans (RDG) approximation results. The results of this study will demonstrate that the MAC and AAE of BC aggregates are not unique, but dependent on the morphological parameters, primary particle size distribution, and the refractive index. Comparison between the computed MAC and available measured MAC at specific wavelengths will also be made.